Intraocular lenses are used to replace a lens in a patient's eye in the case of cataracts. In such instances, it is important to measure the length and refractive error of the patient's eye in order to provide an appropriate replacement for the lens. In many of these cases, however, it is not possible to use standard optical refractometry because the fundus is not visible through the turbid cataract eye lens. Standard optical refractometry suffers from the additional drawback that it does not measure the length of the patient's eye.
Ultrasound biometry is a method that is used to measure the length of the patient's eye. In performing ultrasound biometry, an ultrasound transducer is placed on the cornea of the patient's eye. Next, an ultrasound pulse is emitted by the transducer and is reflected back from the fundus of the eye. As is well known, the time of flight of the pulse depends on the length of, and the index of refraction of the ultrasound pulse in, the patient's eye. Using this information, the length of the patient's eye can be determined. However, there is a drawback in using ultrasound biometry. In particular, in order to receive an echo which is strong enough to be measured with a suitable signal-to-noise ratio, the ultrasound transducer has to be brought into contact with the patient's eye and a special contact jelly has to be used. This is a problem in that the method is invasive and, in many countries, only a medical doctor is permitted to use this method.
An alternative method used to measure the length of the patient's eye makes use of short coherence interferometry and is free from the above-described drawback. This alternative method is described in an article entitled "Optical Coherence Tomography" by A. Fercher, Journal of Biomedical Optics, Vol. 1, No.2, April, 1996, pp. 157-173 (the "Fercher article"). However, this alternative method is limited in that it only measures the length, and not the refractive error, of the patient's eye.
In light of the above, there is a need for a method and apparatus for simultaneously measuring the length and refractive error of an eye, preferably in a non-contact mode.